The present invention relates to a method for determining time errors in connection with analogue-digital conversion of quadrature detected signals with the aid of a test signal.
In the areas of radar technology and telecommunication, as well as in other areas, information exists in the form of carrier waves. To extract the transferred information from these signals, the carrier wave signal is demodulated by down conversion in one or several stages. Because the transferred information exists both in the amplitude as well as in the phase of the signal, a simple envelope detection is not enough. The final down conversion is therefore performed in two parallel mixers in which the signal is mixed with two reference signals with a 90.degree. phase difference.
The result is two orthogonal signals that are a complex representation of the information where both amplitude and phase information are preserved. These signals are normally termed I (In phase) and Q (Quadrature). Because modern signal processing almost exclusively uses digital technology, the I and Q signals are converted to digital form in an analogue-digital converter (A/D-converter), whereafter the signal processing is performed.
As the demands on amplitude similarity and phase orthogonality between I and Q are very high in most applications, extremely high demands are put on similarity between the signals paths of I and Q, among others on the A/D-converters. Apart from high demands on the included components, it is often necessary to also determine, in a suitable way, the differences between the signal paths and, for example, make corrections for them in the subsequent signal processing.
As an example of this, reference is made to European patent application EP 0 490 275. In this document, a doppler radar system is described where, with the use of FFT (Fast Fourier Transform), amplification and phase errors are detected in the signal paths. The size of the errors are computed and corrections are made.
However, time errors can also appear in the signal paths, so that the I and Q signals are A/D-converted at different times. This type of error is not treated in the earlier mentioned, known device.